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=Ribonuclease S=

Ribonuclease S (RNase S) is a modified version of Ribonuclease A, a pancreatic nuclease found in vertebrates. RNase S is synthesized by the proteolytic cleavage of RNase A by subtilisin. Neither of the resulting fragments have ribonuclease activity, but together as the RNase S complex it has full enzymatic activity. As a ribonuclease, RNase S functions to catalyze the hydrolysis of certain internucleotide linkages of RNA. =Structure and Function= RNase S is composed of two fragments: the small fragment, S-peptide (residues 1-20), and the large fragment, S-protein (residues 21-124). These fragments remain tightly bound by non-covalent interactions. The only observed change in covalent structure during the conversion of RNase A to RNase S is the hydrolysis of the peptide bond between the residues 20 and 21. This complex (RNase S) conserves the catalytic activity and native conformation of uncleaved RNase A, but shows a reduced conformational stability. Two hydrophobic residues, methionine 13 and phenylalanine 8, of the S-peptide contribute significantly to the stability of RNase S, while three residues (Phe 8, His 12, and Met 13) seem to be essential for the for the formation of the catalytically active RNase S. It has four disulfide bonds that impose rigidity to the protein. RNase S can form either as a monomer or dimer, which have similar backbone structures except for in the hinge loop region. The dimer has a trans Asn113-Pro114 peptide bond in the hinge loop, whereas the monomer has a cis bond in this position. The RNase S dimer shows significant activity against poly(A)poly(U) sequences and single stranded RNA, similar to the enzymatic activity of RNase A. =Mechanism=

Dimer Formation
Proteolysis of RNase S can activate oligomerization by destabilizing the native state. This occurs via the three dimensional domain-swapping mechanism. In this mechanism two monomers trade structural motifs called swap domains which adopt essentially identical conformations in the monomeric and oligomeric forms. RNase S oligomerizes by swapping C termini, which are not cut by subtilisin.

Dissociation of RNase S Dimers
There are two possible pathways through which RNase S dimers dissociate; however, it is believed that dissociation occurs mainly through pathway 1. Pathway 1 involves the rapid separation and dissociation of the dimers due to a weakened union between the swapped β-strand and the S-protein moiety caused by the separation of S-peptide from one subunit which is the rate limiting step. Whereas, pathway 2 has a rate limiting step of the separation of a swapped β-strand and hinge loop and the accompanying loss of stabilizing interactions. =References=